Abstract

Fertilisation is one of the most cost-effective methods of increasing and maintaining the productivity of Eucalyptus grandis plantations in South Africa. This silvicultural practice can be optimised by using the foliar nutrient ratios measured in plants at maximum growth as a
guideline for fertiliser application. The foliar nutrient ratios present at optimum growth are similar across a wide range of plant species and can be defined in relation to nitrogen (N). This study was conducted to determine whether the proportions of elements relative to N could be refined
specifically for E. grandis through the use of diagnosis and recommendation integrated system (DRIS) norms. The DRIS norms were derived by constructing a data bank of foliar nutrient concentrations and corresponding growth data from published studies on E. grandis. The DRIS norms
for phosphorus (P) and potassium (K) relative to N corresponded to the target values recommended for maintaining optimal nutrient status in other forest species, but the calcium (Ca): N and magnesium (Mg): N ratios were much higher, indicating possible luxury consumption of these elements.
Foliar ratios, expressed relative to 100 N, of 8, 35, 2.5 and 4 for P, K, Ca and Mg appear optimal for the growth of E. grandis. The potential application of this system was illustrated by using the DRIS indices to diagnose the nutrient status of an irrigated and fertilised E. grandis
stand in the KwaZulu-Natal Midlands. In most cases, the diagnosis made using the DRIS indices was confirmed with the optimal ratio approach. The importance of using accurate DRIS norms and correctly interpreting the DRIS indices is emphasised.